| With the increase in grid-connected capacity of new energy sources such as wind power and solar energy,the problems of power dispatching,transmission capacity,power flow control of the power system are becoming more and more prominent.In other words,the safe and stable operation of the power system is facing serious challenges.Certainly,it is able to meet the gradually increasing electric power transmission demand through building new high-voltage and even extra-high-voltage AC or DC transmission lines.However,it is also worthy of further research and development of flexible alternate current transmission system(FACTS)technology for the purpose of flexible,efficient and economical use of existing transmission and distribution assets.Firstly,the development of the SEN transformer(ST)and its variants at home and abroad is reviewed.The ST,as a typical representative of electromagnetic unified current controllers,has received increasing attention in recent years.The advantages and disadvantages of the ST and several other types of power flow controllers are summarized.The current status of research on steady-state and transient models of the ST and the electronic on-load tap-changers(EOLTC)are briefly introduced.Secondly,in order to reveal the internal characteristics of the ST,an electromagnetic analytical model in which the multi-winding coupling in the ST with three-phase three-limb structure is taken into consideration is proposed,which consists of an electromagnetic coupling model and an electrical analytical model.With the help of MATLAB software,by taking a three-phase three-limb model of ST as an example,the electromagnetic analytical results are compared with the existing results from timedomain simulation about the series compensating voltage of ST.The results show that the proposed model can more accurately reflect its internal electromagnetic characteristics through the asymmetry characterization of its magnetic circuit.Thirdly,in order to improve the dynamic response speed of conventional ST,a new extended SEN transformer(EST)topology is proposed based on EOLTC,and a switching transient model of the EST is developed.This model including the steadystate voltage and current of the EOLTC,the evaluation of current and voltage transients during the commutation process,and the dimensioning of the EOLTC and voltage spike protection.The results of the case studies show that the load power factor may affect determining the overlap time of the commutation process of the EOLTC.Furthermore,it is also verified through the time-domain simulation that the application of high-power EOLTC in the ST can increase the speed of operation of the conventional ST.Finally,the design of the protection system within the EST unit is investigated in conjunction with the proposed switching transient model of the EST,which specifically consists of starting circuit and short-circuit protection(SCSCP),overcurrent and overvoltage protection(OP),voltage spike protection(VSP),and atmospheric discharge protection(ADP).Simulation tests were carried out with the help of PSCAD/EMTDC X4.5 software,and the results initially verified the effectiveness of the proposed protection system. |
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